git-write-tree writes garbage on sparc64

In the "next" branch, write_index_ext_header() writes garbage on a
64-bit big-endian machine; the written index file will be unreadable.
I noticed this on NetBSD/sparc64. Reproducible with:

$ git init-db
$ :>file
$ git-update-index --add file
$ git-write-tree
$ git-update-index
error: index uses  extension, which we do not understand
fatal: index file corrupt

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

read-cache.c

@@ -643,7 +643,7 @@ static int ce_write(SHA_CTX *context, int fd, void *data, unsigned int len)
 }
 
 static int write_index_ext_header(SHA_CTX *context, int fd,
-				  unsigned long ext, unsigned long sz)
+				  unsigned int ext, unsigned int sz)
 {
 	ext = htonl(ext);
 	sz = htonl(sz);